metamorphic
River Rock (Granitic Gneiss)
Foliated Granitic Gneiss
Hardness: 6-7 Mohs; Color: Gray with pinkish-orange feldspar and dark biotite/hornblende; Luster: Dull to pearly; Structure: Foliated with visible banding; Cleavage: Poor; Specific Gravity: 2.6-2.9
- Hardness
- 6-7 Mohs
- Luster
- Dull to pearly
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Physical properties
Hardness: 6-7 Mohs; Color: Gray with pinkish-orange feldspar and dark biotite/hornblende; Luster: Dull to pearly; Structure: Foliated with visible banding; Cleavage: Poor; Specific Gravity: 2.6-2.9
Formation & geological history
Formed through high-grade regional metamorphism of igneous granite or sedimentary rocks. This specific specimen has been further shaped by fluvial (river) erosion, giving it a smooth, rounded texture. Many such rocks date back hundreds of millions to billions of years.
Uses & applications
Used in landscaping, decorative riverbeds, construction aggregates, and occasionally as polishing stones for lapidary hobbies.
Geological facts
The banding in gneiss occurs because of high pressure and temperature which causes minerals to recrystallize and separate into layers. River rocks are often called 'cobbles' when they reach this size range.
Field identification & locations
Identify in the field by its smooth, water-worn surface and the presence of alternating light and dark mineral bands. Commonly found in riverbeds, glacial tills, and beaches. Collectors should look for unique banding patterns.
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